Iron alloys, including steel, make up the vast majority of the metals production around the world. Iron and steel development have driven human progress since before the Industrial Revolution forming the backbone of human technological development. In particular, steel has improved the everyday lives of humanity by allowing buildings to reach higher, bridges to span greater distances, and humans to travel farther. Accordingly, production of steel continues to increase over time with a current US production around 100 million tons per year with an estimated value of $75 billion. These steel alloys can be broken up into three classes based upon measured properties, in particular maximum tensile strain and tensile stress prior to failure. These three classes are: Low Strength Steels (LSS), High Strength Steels (HSS), and Advanced High Strength Steels (AHSS). Low Strength Steels (LSS) are generally classified as exhibiting tensile strengths less than 270 MPa and include such types as interstitial free and mild steels. High-Strength Steels (HSS) are classified as exhibiting tensile strengths from 270 to 700 MPa and include such types as high strength low alloy, high strength interstitial free and bake hardenable steels. Advanced High-Strength Steels (AHSS) steels are classified by tensile strengths greater than 700 MPa and include such types as martensitic steels (MS), dual phase (DP) steels, transformation induced plasticity (TRIP) steels, and complex phase (CP) steels. As the strength level increases the trend in maximum tensile elongation (ductility) of the steel is negative, with decreasing elongation at high tensile strengths. For example, tensile elongation of LSS, HSS and AHSS ranges from 25% to 55%, 10% to 45%, and 4% to 30%, respectively.
Steel utilization in vehicles is also high, with advanced high strength steels (AHSS) currently at 17% and forecast to grow by 300% in the coming years [American Iron and Steel Institute, (2013), Profile 2013, Washington, D.C.]. With current market trends and governmental regulations pushing towards higher efficiency in vehicles, AHSS are increasingly being pursued for their ability to provide high strength to mass ratio. The formability of steel is of unique importance for automotive applications. Forecast parts for next generation vehicles require that materials are capable of plastically deforming, sometimes severely, such that a complex geometry will be obtained. High formability steel provides benefit to a part designer by allowing for the design of more complex part geometries facilitating the desired weight reduction.
Formability may be further broken into two distinct forms: edge formability and bulk formability. Edge formability is the ability for an edge to be formed into a certain shape. Edges, being free surfaces, are dominated by defects such as cracks or structural changes in the sheet resulting from the creation of the sheet edge. These defects adversely affect the edge formability during forming operations, leading to a decrease in effective ductility at the edge. Bulk formability on the other hand is dominated by the intrinsic ductility, structure, and associated stress state of the metal during the forming operation. Bulk formability is affected primarily by available deformation mechanisms such as dislocations, twinning, and phase transformations. Bulk formability is maximized when these available deformation mechanisms are saturated within the material, with improved bulk formability resulting from an increased number and availability of these mechanisms.
Bulk formability can be measured by a variety of methods, including but not limited to tensile testing, bulge testing, bend testing, and draw testing. High strength in AHSS materials often leads to limited bulk formability. In particular, limiting draw ratio by cup drawing is lacking for a myriad of steel materials, with DP 980 material generally achieving a draw ratio less than 2, thereby limiting their potential usage in vehicular applications.
Hydrogen assisted delayed cracking is also a limiting factor for many AHSS materials. Many theories exist on the specifics of hydrogen assisted delayed cracking, although it has been confirmed that three pieces must be present for it to occur in steels; a material with tensile strength greater than 800 MPa, a high continuous stress/load, and a concentration of hydrogen ions. Only when all three parts are present will hydrogen assisted delayed cracking occur. As tensile strengths greater than 800 MPa are desirable in AHSS materials, hydrogen assisted delayed cracking will remain problematic for AHSS materials for the foreseeable future. For example, structural or non-structural parts or components used in vehicles, such as bodies in white, vehicular frames, chassis, or panels may be stamped and in the stampings there may be drawing operations to achieve certain targeted geometries. In these areas of the stamped part or component where drawing was done then delayed cracking can occur resulting in scrapping of the resulting part or component.